1. Technical Field
The present invention relates to a magnetic sensor circuit to be connected to a power source having an internal resistor.
2. Background Art
A conventional magnetic sensor circuit is described. FIG. 7 illustrates the conventional magnetic sensor circuit.
In the conventional magnetic sensor circuit, a Hall voltage is generated in a Hall element 61 by a magnetic field applied to the Hall element 61. The Hall voltage is amplified by an amplifier circuit 62. A reference voltage circuit 63 generates a reference voltage Vref. A comparator circuit 64 compares an amplified Hall voltage Vh with the reference voltage Vref. In this case, for example, when the amplified Hall voltage Vh is higher than the reference voltage Vref, an output voltage VOUT is in a high level, whereas when the amplified Hall voltage Vh is lower than the reference voltage Vref, the output voltage VOUT is in a low level (see, for example, Japanese Patent Application Laid-Open No. 2005-260629 (FIG. 5)).
Patent Document 1: JP2005-260629 (FIG. 5)
Incidentally, in seeking a magnetic sensor circuit having low power consumption, there is available a technology as illustrated in FIG. 1 in Japanese Patent Application Laid-open No. 2005-260629 to provide a sample and hold circuit (not shown) at a subsequent stage of the amplifier circuit 62. In this case, the amplified Hall voltage Vh is sampled by the sample and hold circuit during a sample period. During a subsequent comparison period, the sampled voltage Vh is compared with the reference voltage Vref by the comparator circuit 64. The magnetic sensor circuit is controlled to stop the power supply to the Hall element 61 and the amplifier circuit 62 during the comparison period, to thereby reduce the power consumption of the magnetic sensor circuit correspondingly.
A power source 50 includes an internal resistor 51 and an internal power source 52. A power supply voltage VDD of the magnetic sensor circuit drops from an internal power supply voltage VDDPS of the internal power source 52 to a voltage (VDDPS−Rvdd·Idd) based on a resistance value Rvdd of the internal resistor 51 and a current value Idd of current consumption of the magnetic sensor circuit which is caused by mainly the Hall element 61 and the amplifier circuit 62.
In the technology described above, during the sample period, power is supplied to mainly the Hall element 61 and the amplifier circuit 62 to drop the power supply voltage VDD. However, during the comparison period, power is not supplied to the Hall element 61 and the amplifier circuit 62, and hence the power supply voltage VDD hardly drops. Therefore, the comparator circuit 64 compares the voltage Vh sampled under the drop of the power supply voltage VDD with the reference voltage Vref determined in the state in which the power supply voltage VDD hardly drops.
In this case, when a magnetoelectric conversion coefficient of the Hall element 61 is denoted by KH, the following Expressions (21) to (23) hold.VDD=VDDPS−Rvdd·Idd  (21)KH∝VDD  (22)Vref∝VDDPS  (23)
From Expression (22), Expression (24) also holds.Vh∝VDD  (24)
A magnetic detection level Bdet on the detection of a magnetic field applied to the Hall element 61 is expressed by the following Expression (25).Bdet=Vref/KH  (25)
Therefore, from Expressions (22) to (25), the magnetoelectric conversion coefficient KH is proportional to the dropped power supply voltage VDD, and hence the voltage Vh based on the magnetic field is also proportional to the dropped power supply voltage VDD. In contrast, the reference voltage Vref is proportional to the power supply voltage VDD (internal power supply voltage VDDPS) which hardly drops, and hence the magnetic detection level Bdet depends on the resistance value Rvdd of the internal resistor 51.